Resolve "Wiki comments cannot be edited"

What does this MR do and why?

This MR fixes #571700 (closed). When refactoring the save method in !201757 (merged), I missed the fact that the Update mutation requires a slightly different variable format. This MR fixes that by providing the appropriate variable in case the comment is being edited.

Screenshots or screen recordings

No UI changes

How to set up and validate locally

1. Open a wiki page, and create a comment
2. edit the comment
3. save. The edited comment should now be saved correctly.
4. create a reply to that comment.
5. edit the reply.
6. save the edited reply. this should work now, too.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #571700 (closed)